The alteration of the inherent behavior of a system. As an example, ships tend to exhibit significant rolling motions at sea. To dampen these rolling motions, a roll stabilization (feedback) system can be used. Such a system consists of a set of vanes (that is, small wings) extending outward from the hull, below the waterline. By varying the vane incidence angle relative to the hull, a hydrodynamic lift is generated on the vane. The vanes are driven by a feedback system so that the rolling motions are opposed by creating positive lift on one side of the hull and negative lift on the other side.
As a second example, nearly all satellites require some form of stability augmentation to help in keeping the antennas or sensors aligned with receiving equipment on Earth. The stability augmentation is effected by thrusters which receive their commands from a feedback system.
As a third example, stability augmentation systems are used on aircraft. This is usually achieved by a system which controls one or more flight-control surfaces (or engines) automatically without inputs from the pilot. The inherent stability and response behavior of many modern airplanes tends toward low damping or even instability. The physical reasons have to do with the configuration of the airplane and the combination of flight speed and altitude at which the airplane is operated. Several modern fighters and even some transports are intentionally designed with no or little inherent stability. There are a number of reasons for such a design condition. In the case of fighters, excellent maneuverability in combat is essential. By making a fighter intentionally inherently unstable, it is easy to design the control system so that load factors in pull-ups or in turns can be built up rapidly. In the case of transports, the motivation to design for little or no inherent stability is to lower the size of the tail and thereby achieve a reduction in drag and weight. See Airplane
The control exercised by the stability augmentation system contrasts with that exercised by the pilot. The pilot may be connected with the flight-control surface via a direct mechanical link. Alternatively, in many modern airplanes the pilot cockpit control movement is sensed by a position transducer. The output of the position transducer in turn is sent, via a computer-amplifier combination, to a hydraulic actuator, referred to as a servo, which drives the flight-control surface. Command signals which come from the pilot or from the stability augmentation system are sent by wire (fly-by-wire) or by optical conduit (fly-by-light) to the electromagnetic valve. A valve distributes high-pressure hydraulic fluid either to the left or to the right of the piston so that the piston is forced to move. The piston in turn moves the flight-control surface. See Flight controls
With the introduction of fast in-flight digital computers, it has become possible to equip airplanes with so-called full flight envelop protection systems. Such systems are designed to refuse any pilot input which might get the airplane into a flight condition from which recovery is no longer possible. Such systems can easily be arranged to prevent a pilot from rolling a commercial airplane too much or to prevent the pilot from stalling the airplane. Such systems can also be arranged so that loads acting on the wing or tail do not approach dangerously high levels. In that case the system is referred to as a load-alleviation system.